The invention disclosed herein pertains to a mill for reducing sheets of wood pulp to discrete fibers. One use of such fibers is as absorbent material in disposable diapers, sanitary napkins and the like.
All of the fibers in pulp sheets are not full length. Short fibers are characterized as dust or fines. The pulp sheets of some makers have a larger percentage of fines than others.
One of the problems with prior or pre-existing mills is that they break down an additional substantial percentage of the fibers into dust or fines. Long fibers are more absorbent which is desirable. Fines do not hold liquid.
Fines production in known mills has been determined to be largely due to the manner in which the gross particles of pulp are broken down into discrete fibers. The commonly used procedure is to subject the pulp particles to a shearing action. Bladelike members interleaved with each other as they move relative to each other as shears have a tendency to repeatedly shear the fibers to a shorter length which is well known to impair absorption. Known mills that comminute dry pulp particles into fibers have forced the fibers through a screen immediately before they are discharged from the mill. Forcing the fibers through a screen has a tendency to break down the fibers even more whereby to increase fines production. Equally important from the economic point of view is that forcing fibers through a screen requires power which means that the power consumed by such mills is inordinately high.
A mill over which the new mill disclosed herein is an improvement, is described in Canadian Patent No. 1,190,078, dated Jul 9, 1985. This mill is comprised of an involute-shaped housing similar to the housing of a centrifugal fan. A flat faced rotary disk is arranged concentrically with the interior of the housing. The face of the disk has radially extending and angularly spaced apart rows of cutter teeth on it. There is a pulp sheet infeed slot in one side of the housing toward which these cutter teeth project. As the pulp sheet is fed through the slot, its leading edge is pushed into the rotating cutter teeth which initiates breaking the pulp sheet into particles. The periphery of the rotating disk has hammers or wipers on it. There is a short segment concentric to the rotational path of the hammers which has teeth on it that cooperate with the hammers or wipers to break down the pulp particles. Any given pulp particle can experience several orbits around the interior of the housing before being fully comminuted into fibers such as to allow it to pass through a screen which extends over the periphery of about 180.degree. of the rotating disk. The screen assures that no large particles will be discharged from the mill but the perforations in the screen have a propensity to plug or reduce their size at least so that a considerable amount of energy is required to force the fibers through the screen along an involute channel to an outlet. In the patented mill the rows of cutters on the face of the rotary disk act somewhat like fan blades which drive the particles and fibers radially outwardly toward the curved breaker plate segment without very much action taking place on the particles while they are in transit to the breaker plate or the screen. Thus, throughput of the mill is not optimized.
Another known fiberizer mill is described in U.S. Pat. No. 3,815,835, dated Jun. 11, 1974. This mill has a rotating disk which is conical. In other words, its face is beveled radially outwardly from its center so that it is thinner at the periphery than at the center. There is a mosaic of axially extending prongs or cutters projecting from the face of the disk. A substantially corresponding number of stationary cutters or teeth project from a wall of the housing of the mill and they intermesh or interdigitate with the cutters projecting from the rotary disk such that particles captured between the clearance spaces between the rotary and stationary cutters or teeth are sheared as they advance radially outwardly toward an outlet in the housing. The objective of the patented mill is to produce the finest possible particles as is described in the patent. The patented device would be inappropriate for reducing dry pulp to its constituent fibers because the mill is designed for producing extremely fine particles which would be characterized as waste dust or fines in the pulp fiberizing art.
Another type of fiberizer is disclosed in U.S. Pat. No. 3,538,551. In one embodiment a rotary disk having a frusto conical face is used. The face has a multiplicity of axially extending needles against which the edge of a pulp sheet is fed. The needles disintegrated the pulp into fibers. The rotary frusto conical disk does not cooperate with a stationary conical disk as in the present invention.